From 1f3319c83721401e31642c1515bd9be7c567abc1 Mon Sep 17 00:00:00 2001
From: Jason Zaugg <jzaugg@gmail.com>
Date: Wed, 6 Aug 2014 10:02:00 +1000
Subject: SI-8781 Avoid double-expansion under -Ymacro-expand:discard

This mode of macro expansion is used by the presentation compiler to
leave the original macro applications ("expandees") in the type
checked trees, annotated with the types of the expansions.

However, under some circumstances involving implicits, we would
re-expand the macro. If the macro wasn't stable, this could lead
to a type mismatch.

The originally reported problem was with the shapeless
`mkSingletonOps` macro. Its expansion had the type of a freshly-named
class local to the expansion. Upon the re-expansion, a new class
was generated, which lead to errors like:

    client/Client.scala:4: error: type mismatch;
     found   : fresh$macro$2
     required: fresh$macro$1

This commit suppressed re-expansion of the expandee by use of
the existing, tree attachment driven mechanism.
---
 src/compiler/scala/tools/nsc/typechecker/Macros.scala |  5 ++++-
 test/files/pos/t8781/Macro_1.scala                    | 13 +++++++++++++
 test/files/pos/t8781/Test_2.flags                     |  1 +
 test/files/pos/t8781/Test_2.scala                     |  5 +++++
 4 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 test/files/pos/t8781/Macro_1.scala
 create mode 100644 test/files/pos/t8781/Test_2.flags
 create mode 100644 test/files/pos/t8781/Test_2.scala

diff --git a/src/compiler/scala/tools/nsc/typechecker/Macros.scala b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
index 9c22688581..33d3432ae2 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Macros.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
@@ -576,7 +576,10 @@ trait Macros extends MacroRuntimes with Traces with Helpers {
                 // also see http://groups.google.com/group/scala-internals/browse_thread/thread/492560d941b315cc
                 val expanded1 = try onSuccess(duplicateAndKeepPositions(expanded)) finally popMacroContext()
                 if (!hasMacroExpansionAttachment(expanded1)) linkExpandeeAndExpanded(expandee, expanded1)
-                if (settings.Ymacroexpand.value == settings.MacroExpand.Discard) expandee.setType(expanded1.tpe)
+                if (settings.Ymacroexpand.value == settings.MacroExpand.Discard) {
+                  suppressMacroExpansion(expandee)
+                  expandee.setType(expanded1.tpe)
+                }
                 else expanded1
               case Fallback(fallback) => onFallback(fallback)
               case Delayed(delayed) => onDelayed(delayed)
diff --git a/test/files/pos/t8781/Macro_1.scala b/test/files/pos/t8781/Macro_1.scala
new file mode 100644
index 0000000000..ecd9c5e8d5
--- /dev/null
+++ b/test/files/pos/t8781/Macro_1.scala
@@ -0,0 +1,13 @@
+import scala.reflect.macros.whitebox.Context
+import language.experimental.macros
+
+object Macros {
+  def impl(c: Context) = {
+    import c.universe._
+    val name = TypeName(c.freshName())
+    q"class $name extends T; new $name"
+  }
+  def fresh: Any = macro impl
+}
+
+trait T
diff --git a/test/files/pos/t8781/Test_2.flags b/test/files/pos/t8781/Test_2.flags
new file mode 100644
index 0000000000..24e2109690
--- /dev/null
+++ b/test/files/pos/t8781/Test_2.flags
@@ -0,0 +1 @@
+-Ymacro-expand:discard -Ystop-after:typer
diff --git a/test/files/pos/t8781/Test_2.scala b/test/files/pos/t8781/Test_2.scala
new file mode 100644
index 0000000000..3ca6406599
--- /dev/null
+++ b/test/files/pos/t8781/Test_2.scala
@@ -0,0 +1,5 @@
+object Test {
+  implicit class RichT(t: T) { def augmented = "" }
+
+  Macros.fresh.augmented
+}
-- 
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